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            <h3 class="mb-0">MySQL中INNER JOIN详解</h3>
            
            <small class="text-muted">发布于 2019-05-19</small>
        </div>

        <article>
            <h2 id="前言">前言</h2>
<p>MySQL中联合查询有join、inner join、left join、right join、full join这五种组成。join和inner join是相等的。一般涉及到多张数据表之前的关联查询，必须要用连接操作符。本次来讲讲inner join的使用。</p>
<h2 id="使用">使用</h2>
<p>首先，为了能够形象的展示Inner join的效果，创建两张相关联的数据表，学生表和成绩表</p>
<p>学生表创建语句如下：</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-sql" data-lang="sql"><span style="display:flex;"><span><span style="color:#66d9ef">create</span> <span style="color:#66d9ef">table</span> student(
</span></span><span style="display:flex;"><span>    id int <span style="color:#66d9ef">not</span> <span style="color:#66d9ef">null</span> auto_increment,
</span></span><span style="display:flex;"><span>    name varchar(<span style="color:#ae81ff">64</span>) <span style="color:#66d9ef">not</span> <span style="color:#66d9ef">null</span> <span style="color:#66d9ef">default</span> <span style="color:#e6db74">&#39;&#39;</span> <span style="color:#66d9ef">comment</span> <span style="color:#e6db74">&#39;姓名&#39;</span>,
</span></span><span style="display:flex;"><span>    <span style="color:#66d9ef">primary</span> <span style="color:#66d9ef">key</span>(id)
</span></span><span style="display:flex;"><span> ) charset<span style="color:#f92672">=</span>utf8mb4 <span style="color:#66d9ef">comment</span> <span style="color:#e6db74">&#39;学生表&#39;</span>;
</span></span></code></pre></div><p>成绩表创建语句如下：</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-sql" data-lang="sql"><span style="display:flex;"><span><span style="color:#66d9ef">create</span> <span style="color:#66d9ef">table</span> score(
</span></span><span style="display:flex;"><span>    id int <span style="color:#66d9ef">not</span> <span style="color:#66d9ef">null</span> auto_increment,
</span></span><span style="display:flex;"><span>    student_id varchar(<span style="color:#ae81ff">64</span>) <span style="color:#66d9ef">not</span> <span style="color:#66d9ef">null</span> <span style="color:#66d9ef">default</span> <span style="color:#e6db74">&#39;&#39;</span> <span style="color:#66d9ef">comment</span> <span style="color:#e6db74">&#39;学生id&#39;</span>,
</span></span><span style="display:flex;"><span>    chinese varchar(<span style="color:#ae81ff">4</span>) <span style="color:#66d9ef">not</span> <span style="color:#66d9ef">null</span> <span style="color:#66d9ef">default</span> <span style="color:#e6db74">&#39;&#39;</span> <span style="color:#66d9ef">comment</span> <span style="color:#e6db74">&#39;中文&#39;</span>,
</span></span><span style="display:flex;"><span>    <span style="color:#66d9ef">primary</span> <span style="color:#66d9ef">key</span>(id)
</span></span><span style="display:flex;"><span> ) charset<span style="color:#f92672">=</span>utf8mb4 <span style="color:#66d9ef">comment</span> <span style="color:#e6db74">&#39;学生表&#39;</span>;
</span></span></code></pre></div><p>学生表的数据如下：</p>
<table>
  <thead>
      <tr>
          <th>ID</th>
          <th>NAME</th>
      </tr>
  </thead>
  <tbody>
      <tr>
          <td>1</td>
          <td>小明</td>
      </tr>
      <tr>
          <td>2</td>
          <td>小华</td>
      </tr>
      <tr>
          <td>3</td>
          <td>小丽</td>
      </tr>
      <tr>
          <td>4</td>
          <td></td>
      </tr>
  </tbody>
</table>
<p>成绩表数据如下</p>
<table>
  <thead>
      <tr>
          <th>ID</th>
          <th>STUDENT_ID</th>
          <th>CHINESE</th>
      </tr>
  </thead>
  <tbody>
      <tr>
          <td>1</td>
          <td>1</td>
          <td>89</td>
      </tr>
      <tr>
          <td>2</td>
          <td>2</td>
          <td>78</td>
      </tr>
      <tr>
          <td>3</td>
          <td>3</td>
          <td>64</td>
      </tr>
      <tr>
          <td>4</td>
          <td>5</td>
          <td>96</td>
      </tr>
      <tr>
          <td>问题1: 查询有成绩的学生信息</td>
          <td></td>
          <td></td>
      </tr>
  </tbody>
</table>
<h3 id="步骤1">步骤1</h3>
<p>查询两个表连接的数据</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-sql" data-lang="sql"><span style="display:flex;"><span><span style="color:#66d9ef">select</span> <span style="color:#f92672">*</span> <span style="color:#66d9ef">from</span> student <span style="color:#66d9ef">inner</span> <span style="color:#66d9ef">join</span> score
</span></span></code></pre></div><p>结果如下：
<img src="https://ueyao.github.io/image-hosting/blog/2019/2019-05-19-7.44.24.png" alt="查询结果"></p>
<p>这个结果有许多相同的数据，这个情况叫做笛卡尔积，如何避免笛卡积，需要在内连接查询上添加查询条件就可以解决这个问题。</p>
<h3 id="步骤2">步骤2</h3>
<p>查询两个表连接的数据带条件</p>
<div class="highlight"><pre tabindex="0" style="color:#f8f8f2;background-color:#272822;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-sql" data-lang="sql"><span style="display:flex;"><span><span style="color:#66d9ef">select</span> s.id,s.name,sc.chinese  <span style="color:#66d9ef">from</span> student s <span style="color:#66d9ef">inner</span> <span style="color:#66d9ef">join</span> score sc <span style="color:#66d9ef">on</span> s.id <span style="color:#f92672">=</span> sc.student_id
</span></span></code></pre></div><p>查询的结果如下：
<img src="https://ueyao.github.io/image-hosting/blog/2019/2019-05-19-7.57.07.png" alt="查询结果"></p>
<p>从上图中，可以看到没有小花的成绩数据，因为内连接只会显示两个表中都有的数据。用集合的方式来说就是取两个表数据的交集。</p>
<p>总结
开发中常常会查询相关联的数据，并且查询中只保留两表中共有的数据，这样就可以使用inner join来关联查询。</p>

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